The present invention relates to a radiation detector for use in a computerized tomography apparatus.
One conventional computerized tomography apparatus displays a picture of the cross section of an object, such as a human body, by using radiation such as X-rays.
This conventional apparatus, as shown in FIG. 1 or 2, is made up of an X-ray source 1 for radiating a fan-shaped X-ray beam FX and an X-ray detector 2 having an array of X-ray sensing cells, which is disposed in opposition to the X-ray source 1. An object P is disposed between both the devices 1 and 2. FIG. 2 illustrates the situation when an expanding angle .theta.2 between boundary X-rays is narrower than angle .theta.1 of FIG. 1. The source 1 and the detector 2 rotate about the object P along the same peripheral locus, in the same direction, and at the same angular velocity, thereby collecting X-ray projection data of the cross section of the object from every angular location of the object. The collected data is converted into an electrical signal which in turn is analyzed by a computer to calculate absorption indices of the X-rays at every location on the cross section of the object. A picture of the object's cross section is reconstructed by providing tone values corresponding to the absorptance to the display section. The apparatus thus constructed can provide a clear tomogram for soft to hard organisms.
The X-ray detector 2 has a number of sensing cells each consisting of two bias electrode plates and a signal electrode plate which are alternately disposed and filled with ionizable gas, for example, xenon, at high pressure. The X-ray transmitted through the object P projects into every cell constituting an ion chamber where the X-ray energy is detected as an ionization current. The ionization current of each X-ray path (a path connecting the source 1 and the sensing cell) is integrated with respect to time. The integrated value of current is discharged through a discharge circuit of a given time constant. The discharge time is then used for the X-ray tomographying data in each X-ray path. In this way, when the data collection at one position on the peripheral locus is completed, those devices in advance of the next position effect similar data collection.
An example of a radiator detector of the multichannel type is illustrated in FIGS. 3 to 6. A body 3 of the detector has a cavity 4 for accommodating a number of electrodes and an X-ray permeable window 5 in which the side wall on the incident side is partially thinned when compared to the remaining wall. The side wall extends through to an expanding angle .theta. of the fan-shaped X-ray beam so as to enable the X-ray's energy to reach the internal sensing cells in a sufficient amount. The cavity 4 containing the sensing cells is covered with a cover 6 and filled with ionizing gas, e.g. xenon, at high pressure. Further, in the cavity 4, signal electrode plates 10 for signal sensing and bias electrode plates 11 for applying high voltage are alternately disposed, as shown in FIG. 5. These plates 10 and 11 are firmly fitted at the upper and lower ends in the corresponding grooves of support members 12, ensuring that they exhibit the same given intervals or pitches. One signal electrode plate 10 and two bias electrode plates 11 located on both sides of the former make up a sensing cell. A number of sensing cells are housed in the cavity 4 of the body 3, as shown in FIG. 6. The bias electrode plates 11 are connected to a single lead wire 14 for high voltage application. The signal electrode plates 10 are connected to each lead wire 13 for leading signals from the cells to the exterior.
In the computerized tomography apparatus, the radiation source and the radiation detector must be located at a given location with high precision in order to obtain an excellent picture of the cross section of the object. Specifically, the sensing cells of the radiation detector must be arranged so as to sufficiently cover the expanding angle .theta. of the fan-shaped radiation from the radiation source. Further, the sensing cells must be arranged in parallel with radial lines extending from the radiation source so as to effectively detect the radiation. Generally, the sensing cells are designed and prefabricated so that their openings are directed towards the radiation source. Therefore, it is acceptable to merely set the focal point of the radiation source to coincide with the cross point of each cell in the direction of their openings. When there is an error in the positioning of the radiation source, the opening direction of the sensing cell deviates from the direction of the incoming radiation. As a result, the apparatus incorrectly detects the energy of the incident radiation to provide an artifact as a virtual image in the picture formed. For this reason, it is necessary to ensure accurate alignment in the optical system including the radiation source and the detector. Nevertheless, there has been no method to accurately measure an amount of the positional deviation of the radiation source from its correct position. The positioning of the radiation source depends solely on the operator's skill and his experiance. Therefore, the positioning work is difficult and time-consuming.